Injection molding of medium voltage pole parts, for example, to cover them with a thermoplastic housing, is well known. In contrast to the use of duroplastic material for a resin, the proceeding pressure for thermoplastic material during the molding is higher. An advantage of a thermoplastic housing is that the manufacture times are shorter than the manufacture times for epoxy resin. This is because epoxy resin needs longer curing periods and has slow temperature curves.
Thermoplastic material only has to become solid by cooling down.
Injection molding technology for thermoplastic pole parts is used only with a single injection gate or opening.
Furthermore, due to inlays which are sensitive to pressure such as vacuum-interrupters, for example, injection molding is only possible up to certain pressure limits.
The filling of a cavity depends on the behavior and the properties of thermoplastic materials, in scope of their resulting viscosity.
Drawbacks of known processes include the following:
The filling pressure decreases along the flow path to a low pressure at the end, caused by the viscosity of the thermoplastic material. This finally results in filling problems.
The maximum filling pressure inside the mold increases with the height of the pole part or the pole part arrangement along the flow path.
Geometric constraints require inhomogeneous wall thickness along the flow path, and therefore voids occur, which may also result in incomplete filling at dedicated pressures.
The introduction of geometric reinforcement elements such as fins for the strength and stiffness of a pole part is almost impossible due to the ineligible increase of cavity pressure.
Injection molding of material with increased viscosity is with that known technology not possible.
The manufacture time cycles for thermoplastic covered pole parts are quite short, so dynamic effects like the viscosity of the liquid hot thermoplastic material occur during the process.